The meniscus is a crescent-shaped fibrocartilage tissue paired in both the medial and lateral compartments of the knee between the femur and the tibia. Macroscopically, the menisci are C-shaped (semi-lunar) tissues which feature a triangular (wedge shaped) cross-section and cover and separate the tibial plateau from the femoral condyles providing a bearing surface within the joint. The meniscus plays a critical role in load transmission, stability and in reducing contact stresses in the knee joint which is attributed to the unique shape and microstructure of the tissue. The microstructure of the meniscus is largely comprised of type I collagen fibers and the spatial orientation of these collagen fibers are highly functionalized in order to provide the meniscus' unique mechanical properties. Specifically, the meniscus features circumferential bundles of collagen fibers embedded within a hydrated matrix which acts to bear circumferential hoop stresses. When the meniscus is axially loaded, the meniscus tends to displace radially out of the joint center due to its wedge shaped cross-section. Extrusion is, however, resisted as the meniscus is anchored both posteriorly and anteriorly in the tibia via the meniscal horns, The circumferential arrangement of type I collagen fibers throughout the meniscus as well as the meniscal horns give rise to circumferential, or hoop, stresses which resist radial displacement. The meniscus thus converts vertical load into hoop stresses, thereby reducing contact stresses a mechanism known as the ‘hoop stress mechanism’.
Injuries to the meniscus are the most common injuries requiring surgical intervention in orthopaedic practice with approximately one million surgeries occurring in the United States every year. Traumatic tears occur in young, active individuals and are more common in the medial meniscus, usually as a result of rotational forces applied to a flexed knee, while degenerative meniscal tears tend to occur in middle aged and older individuals, as a result of the weakening of the tissue and hence the reduction in mechanical properties.
With increased meniscal injuries, meniscal repair has become a standard procedure, and the traditional treatment of partial or full resection which was previously identified as the gold standard, is now understood to have deleterious effects and should be performed as sparingly as possible. Replacement of the damaged meniscal tissue with an implant aims to restore the knee biomechanics and might avoid articular cartilage degeneration and the onset of osteoarthritis. Different types of meniscal substitutes, such as allografts, collagen based materials, and synthetic biodegradable scaffolds, have been used in experimental and clinical studies. Allograft menisci suffer from problems relating to availability, size matching, cost, and risk of disease transmission. Additionally both allografts and collagen scaffolds experience remodeling after implantation causing shrinkage and reduced mechanical strength. Synthetic biodegradable scaffolds lack durability under knee loading conditions and also vary in the body response to the implant and the quality of the tissue formed. Synthetic biodegradable scaffolds also require the native tissue to be present (typically as a rim) for not only attachment but also as source of autologous cells which can seed the implant. Mimicking the microstructure and mechanical property distribution of the meniscus improves the chances for restoring the contact pressures along the tibial plateau during normal activities, and preventing the onset of osteoarthritis following injury. Accordingly, an improved anatomically designed meniscus implantable device is needed to replicate the mechanical properties distribution within the native meniscus.
Prosthetic meniscus implants are a novel treatment method to repair meniscal lesions. To date, there is no meniscus substitute that takes into account the microstructure of the native meniscus. Implantable devices that can restore and replace the function of the meniscus would be of great interest for orthopaedic applications given the clear indication of the unmet need in the area. The challenge in developing a meniscal replacement with biomechanical characteristics close to that of the native meniscus lies in the construction and design choice of material. Accordingly, it is an object of the present invention to provide an improved implantable device for treating meniscal injuries through replacement with an anatomically designed meniscus implant.